In commonly owned German published specification P 24 39 228.7-12 a jaw clutch has been described which comprises a central body secured to a shaft between two idler gears which are provided with respective sets of jaw teeth selectively engageable by a toothed ring that is axially slidably on the central body but nonrotatable with reference thereto. In order to prevent such interengagement when the shaft and the gear to be coupled therewith turn at different speeds, the central body is flanked by a pair of blocking rings which are limitedly rotatable with reference to that body between two extreme relative angular positions into which either blocking ring is frictionally entrained by the adjoining idler gear when the speed of the latter is either greater or less than that of the central body. A coupling engagement can therefore take place only upon temporary synchronization of the two rotations, i.e. at the instant when an acceleration or deceleration of either the shaft or the gear moves the corresponding blocking ring from its previous limiting position toward the opposite end of its range past an intermediate position in which that ring is ineffectual to prevent an axial shift of the ring gear to the side of the synchronously rotating idler gear. The temporary synchronization necessary for the clutching operation is generally brought about by a change in engine speed through a controlled depression or release of the vehicular accelerator pedal. Such clutches are particularly useful for heavy-duty utility vehicles with four or more speeds established by manual gear shifting.
When the engine is started up from standstill, the jaw clutch is usually disengaged. With the vehicle either at standstill or rolling very slowly, e.g. on a slope, the clutch body and the idler gear -- either of which may be driven from the engine -- have a relative speed such that the blocking ring is entrained into one of its limiting positions. In order to facilitate coupling, this relative speed would have to be reversed as described above; since, however, the engine can rotate only unidirectionally, such a speed reversal is not possible under these circumstances. To remedy this deficiency, it has already been proposed to provide each blocking ring with a restoring spring designed to hold that ring in its intermediate, nonblocking position when the jaw clutch is disengaged, i.e. when its slidable ring gear is in its axial midposition. A drawback of this solution is that the entrainment of the blocking ring into either of its operating positions requires the frictional transmission of higher torques, with increased danger of slippage and resulting untimely contact between the relatively rotating sets of jaw teeth. A restoring spring, furthermore, is inherently unable to move such a blocking ring into a precisely centered position within its range of relative rotation.